This invention deals with a problem encountered in the manufacture of loose tube fiber optic cable. More particularly, it involves the problem of excess fiber length (EFL) control during the manufacture of the loose tube for optical fibers. EFL (expressed as a percentage) is the length of optical fiber in a loose buffer tube, less the length of the buffer tube, divided by the length of the buffer tube times 100. Excess fiber lies loosely coiled in the buffer tube; however, when the buffer tube itself is stretched, either by thermally induced expansion forces or by mechanical means, the excess fiber permits such buffer tube stretching without putting any tensile forces on the fibers themselves. Excessive EFL is undesirable because of bending restrictions on the fiber over the temperature range at which a cable, made in part from the buffer tubes, encounters in operation.
The prior art manufacture of loose tube buffered optical fibers involves the following steps performed essentially simultaneously: (a) applying a filling compound to one or more optical fibers; and (b) extruding around the filling compound and optical fibers a plastic buffering tube of a diameter that permits the optical fibers to be loosely disposed in the buffer tube to the extent that the fiber(s) can move about within the confines of the buffer tube's interior. Subsequently, the fiber is coupled to the interior of the buffer tube and then cooled where the EFL is achieved. In some fiber optic cable containing loose buffer tubes, zero percent (0%) EFL is desired. In others, anywhere from 0.1 to 0.7 percent EFL is required, depending on the cable design. This invention is directed towards controlling the EFL of optical fibers in a loose buffer tube by controlling the cooling of the buffer tube as it is cooled from its just-extruded to a predetermined elevated temperature. The tube is solidified to the extent that it can be traversed about a rotatable member, where coupling takes place. Thereafter, cooling to ambient temperatures results in the EFL.
In practice, positive EFL's are accomplished by variations in cooling water temperature using certain prior art machinery employed to manufacture loose tube fiber optic cable. The water quench point and dwell time of the tube in the water and/or wrap around sheaves are some of the variables used. The mechanics behind these particular variations involve having a controlled and elevated tube temperature at the point where the optical fibers become coupled to the interior of the buffer tubes. Such coupling is either a mechanical fixation of the fibers to the interior wall of the tube, arising out of the filling compound itself or actual contact and mechanical fixing of the fiber to the interior of the buffer tube without the aid of the filling compound. It may also occur as a result of a combination of these events. In any event, a coupling is where the tube and the fiber travel at the same speed, one is coupled with the other. One prior art method of achieving fiber coupling is to wrap the tube with fiber in it around a sheave. Thermoshrinkage from the elevated temperature to ambient temperature after the fibers have been coupled yields the excess fiber length within the tube.
There are two types of extrusion techniques used in present prior art optical fiber buffer tube manufacture, horizontal and vertical. The vertical type is the preferred. Controlled cooling in a vertical extrusion process is quite difficult because cooling water flow and the buffer tube are moving in the same direction. Breaking contact between cooling water and a cooling buffer tube in a vertical extrusion process cannot be achieved simply by an out flow at the end of a trough, as is the case in horizontal extrusion. When horizontal extrusion is used, water can simply flow out the ends of a horizontal trough and downward off the tube. Thus, there arose a need for controlled water cooling of a fiber optic loose buffer tube using a vertical cooling apparatus. This need required control over the exact point where the water contacted the tube and cooling began in combination with an exact point where water cooling of the tube stopped. The amount of cooling required also needed to be variable in order to achieve corresponding subsequent variations in EFL. Toward this end, EFL's of 0.006% to 0.6976% have been achieved by this invention.